Bead polymers obtained by suspension polymerization of a mixture of a monovinyl monomer and a polyvinyl monomer in water have been used, for example, as a base polymer for ion-exchange resin. In this case, the bead polymers are preferred to have a larger particle size in the viewpoint of water permeability in case of filling a water passing column with the ion-exchange resin.
For example, a process for obtaining a bead polymer by suspension polymerizing using acrylic or methacrylic acid ester as a monovinyl monomer and divinylbenzene as a polyvinyl monomer has been known. According to this process, bead polymers having a comparatively large particle size are obtained by controlling polymerization conditions, for example, by slowly stirring the polymerization bath. However, when the bead polymers obtained by this process are used as an ion-exchange resin, it is required to previously hydrolyze carboxylic acid ester groups in the polymer.
On the other hand, in case of using acrylic acid or methacrylic acid as the monovinyl monomer, the process is simplified because the resulting bead polymers are not required to hydrolyze. However, in case of suspension polymerization of acrylic acid or methacrylic acid together with divinylbenzene in water, the polymerization is generally carried out by adding a salt such as sodium chloride to an aqueous phase to reduce the solubility of monomers in water (hereinafter referred to as "salting-out polymerization"), because acrylic acid or methacrylic acid has a high solubility in water. In this process, CMC which has a good solubility is in general suitably used as a dispersing agent, because there is the possibility of precipitating the dispersing agent in the system if the dispersing agent used does not have a good solubility.
However, in case of the salting-out polymerization using acrylic acid or methacrylic acid as the monovinyl monomer and CMC as the dispersing agent, it is extremely difficult to obtain bead polymers having a large particle size, even if the polymerization condition is controlled. Namely, in case of the salting-out polymerization, a dispersing state of the oil-in-water emulsion cannot be kept if the emulsion is stirred too slowly, due to the large difference of specific gravity between the aqueous phase containing a large amount of the salt and the monomer phase.
Further, in case of suspension polymerization using styrene as the monovinyl monomer, divinylbenzene as the polyvinyl monomer and CMC as the dispersing agent, it has been known to add an organic compound having a specific gravity lower than that of monomers such as toluene to the system to obtain porous bead polymers. However, according to this process, it is extremely difficult to obtain polymers having a large particle size as similar to the case of using acrylic acid or methacrylic acid, due to the large difference of specific gravity between the aqueous phase and the monomer phase.